Feedbacks between coupled subglacial hydrology and glacier dynamics



On most glaciers and ice sheet outlets the majority of motion is due to basal slip, a combination of basal sliding and bed deformation. The importance of basal water in controlling sliding is well established, with increased sliding generally related to high basal water pressure, but the details of the interactions between the ice and water systems has not received much study when there is coupling between the systems. Here we use coupled subglacial hydrology and ice dynamics models within the Community Ice Sheet Model to investigate feedbacks between the ice and water systems. The dominant feedback we find is negative: sliding over bedrock bumps opens additional cavity space, which lowers water pressure and, in turn, sliding. We also find two small positive feedbacks: basal melt increases through frictional heat during sliding, which raises water pressure, and strain softening of basal ice during localized speedup causes cavities to close more quickly and maintain higher water pressures. Our coupled modeling demonstrates that a sustained input of surface water to a distributed drainage system can lead to a speedup event that decays even in the absence of channelization, due to increased capacity of the system through opening of cavities, which is enhanced through the sliding-opening feedback. We find that the negative feedback resulting from sliding-opening is robust across a wide range of parameter values. However, our modeling also argues that subglacial channelization is required to terminate speedup events over timescales that are commensurate with observations of late summer slowdown on mountain glaciers.